The present invention relates to a far-infrared emitter of high emissivity and corrosion resistance and a method for the preparation thereof. More particularly, the invention relates to a stainless steel-made far-infrared emitter having a high emissivity approximating that of a black body and excellent corrosion resistance suitable as a heater element in room heaters and drying or heating apparatuses utilizing far-infrared rays as well as a method for the preparation thereof.
As is well known, far-infrared rays have a characteristic of easily penetrating human bodies and various kinds of organic materials so that room heaters utilizing far-infrared rays are advantagesous in respect of the high efficiency of heat absorption in the depth of the human body and far-infrared drying or heating ovens can be advantageously used for drying of paint-coated surfaces or heating of various kinds of food by virtue of the rapidness of heating.
Several metal oxides such as zirconium oxide, aluminum oxide, silicon dioxide and titanium dioxide are known to emit far-infrared rays with a high efficiency at high temperatures so that many of the far-infrared emitters currently in use are manufactured from a ceramic material mainly composed of one or more of these metal oxides or by providing a metal-made substrate with a ceramic coating layer composed of these metal oxides. Such a ceramic-based far-infrared emitter, however, is practically defective in respect of the fragility to be readily broken by shocks and lack of versatility to the manufacture of large-sized emitters. Metal-based ceramic-coated far-infrared emitters are also not without problems because the ceramic coating layer is liable to fall during use off the substrate surface in addition to the expensiveness of such an emitter.
In view of the above mentioned problems in the ceramic-based far-infrared emitters, many proposals have been made for metal-made heat radiators of infrared emitters. For example, Japanese Patent Publication 59-7789 discloses a heat radiator made of an alloy of nickel and chromium, iron and chromium or iron, chromium and nickel provided with a black oxide film on the surface mainly composed of an oxide of chromium formed by the oxidation at a high temperature. Japanese Patent Publication 59-28959 discloses a stainless steel-made infrared heater element provided with an oxide surface film having a thickness of 1 to 10 .mu.m formed by an oxidation treatment at a high temperature of 700.degree. C. or higher. Japanese Patent Publication 60-1914 discloses an infrared-radiating heater element made of a highly heat resistant alloy such as incoloy and subjected to an oxidation treatment at a high temperature of 800.degree. C. or higher. Further, Japanese Patent Kokai 55-6433 discloses a stainless steel-made radiator provided with an oxide surface film formed by a wet process after roughening of the surface to have a surface roughness of 1 to 10 .mu.m.
While it is desirable that a far-infrared emitter has an emissivity as high as possible, the above described ceramic-based or stainless steel-based emitters have an emissivity rarely exceeding 0.9 or, in most cases, 0.8 or smaller. Far-infrared emitters usually utilize the far-infrared rays emitted from the emitter body at a temperature in the range from 100.degree. to 500.degree. C. As is understood from the Planck's law of radiation distribution, an emitter of low emissivity can emit a far-infrared radiation identical with that from an emitter of higher emissivity only when it is heated at a higher temperature. Needless to say, a larger energy cost is required in order to heat an emitter at a higher temperature. Moreover, certain materials are susceptible to degradation when exposed to a radiation of shorter wavelength such as near-infrared and visible rays so that heat radiators used for such a material are required to emit far-infrared rays alone and the far-infrared emitter should be kept at a relatively low working temperature not to emit radiations of shorter wavelengths. Accordingly, it is eagerly desired to develop a far-infrared emitter having a high emissivity even at a relatively low temperature.
Apart from the above described problem in the emissivity, stainless steel-made far-infrared emitters in general have another problem of relatively poor corrosion resistance. Namely, the working atmosphere of a far-infrared emitter is sometimes very corrosive. For example, a large volume of water vapor is produced when a water-base paint is dried or food is heat-treated with a far-infrared emitter to form an atmosphere of high temperature and very high humidity. When the working hours of such a heating furnace come to the end of a working day, the furnace is switched off and allowed to cool to room temperature so that the water vapor in the atmosphere is condensed to cause bedewing of the surface of the stainless steel-made far-infrared emitter. Thus, it is usually unavoidable that rusting of the stainless steel-made far-infrared emitter starts within a relatively short time as a consequence of the repeated cycles of heating and bedewing. Once rusting has started, it would be before long that scale of the rust comes off the surface to enter the food under the heat treatment or to adhere to the fabric material under drying so that the heating furnace can no longer be used without entirely replacing the far-infrared emitter elements in order to obtain acceptable products.